How to set the heavy load and light load protection value of the electric motor? Read it and understand
Introduction
Electric motors serve as the backbone of many industrial operations, powering everything from conveyor belts to water pumps. They operate under varying conditions, sometimes running under heavy loads and at other times, lighter ones. Setting the right protection values for these disparate scenarios—known as heavy load and light load protection—can make all the difference in how efficiently and safely an electric motor runs. Failure to do so could not only jeopardize the motor but also pose serious risks to both the machinery it powers and the personnel who operate it.
In this article, we will delve into the essential steps for setting the heavy load and light load protection values of electric motors. We will discuss why it's critical to accurately set these parameters, what tools and knowledge you'll need beforehand, and provide a step-by-step guide on actually setting these values. Whether you are an engineer, a maintenance technician, or simply someone interested in understanding the workings of electric motors, this guide aims to equip you with the know-how to ensure that your motors run efficiently, safely, and with a longer lifespan.
A few years ago, I saw an engineer solemnly instructing his subordinates to set the overload protection parameters based on the starting load of a three-phase asynchronous motor.
He emphasized that they must understand whether the motor is started under light or heavy load, otherwise there would be problems because heavy-load starting current can be very high.
At that time, I wanted to point out that this was a misunderstanding but was afraid of offending people so I let it go. Later on, I found out that many electrical engineers have this kind of misconception nowadays and I really don't know how they learned in school. Of course, they may not even be from this field.
Actually, the no-load, light-load, and heavy-load starting currents of three-phase asynchronous motors are the same. One cannot arbitrarily adjust the overload protection setting values.
This is because the start-up torque logic of a motor is as follows: Me=KmфIcosφ For a motor, the structural constant Km, magnetic flux ф, and power factor cosφ are all fixed.
Therefore, the starting current I will not change; only the starting time will be different with shorter times for light loads and longer times for heavy loads.
Electric friends: The locked-rotor current of the motor is fixed, but the curve from locked-rotor to normal standard speed during heavy-load starting (such as fans and belt conveyors) and light-load starting (such as centrifugal pumps) is different. This means that the start-up time mentioned by teachers.
The protection of the motor should be set based on its rated current, and electronic protection can set protection parameters such as start-up time according to load characteristics.
For thermal relay protection settings, if there is testing equipment available, it's best to adjust them offline before installation and commissioning.
The test site should have a similar or same room temperature as the operating site.
As for whether to use voltage reduction starting measures, it depends on the carrying capacity of the power supply system. For example, all three phase motors below 180KW in large petrochemical plants use full-voltage starting.
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